1. Field of the Invention
The present invention relates to a drip detecting device and also to a drip alarming device and a drip rate control device which incorporate the drip detecting device.
2. Description of the Related Arts
As shown in FIG. 5, a known drip detecting device has a combination of a light-emitting diode 51 and a photodiode (or phototransistor) 54. A light emitted from the light-emitting diode 51 is collimated into a beam through the lens 52 which is introduced to a lens 53 through a drip cylinder, the lens 53 converging the light onto the photodiode or phototransistor 54. The drips are detected by the photodiode or phototransistor 54 through a change in the quantity of light which is caused by passage of the drip through the drip cylinder. Namely, as shown in FIG. 6A, the passage of the droplet, i.e., drip, is confirmed when the output level of the photodiode 54 has exceeded a predetermined threshold. However, since only one semiconductor element is used at the light-receiving side so as to receive converged light, the accuracy of detection tends to be degraded during long use. Namely, the quantity of light received by the light-receiving element is progressively reduced by deposition of fine liquid droplets to the inner surface of the drip cylinder due to splashing of liquid or by clouding of the wall of the drip cylinder due to change in temperature. In consequence, the difference between the output voltage level during a drip passing through the drip cylinder and the output voltage level with no drip becomes small to reduce the S/N ratio as shown in FIG. 6B, making it difficult to accurately detect the drips. In addition, detection error is often caused due to incidence of strong external disturbance light such as solar light into the light-receiving element.